Historically, brain neurosurgery has begun with entry into the cranium by removal of parts of the bones making up the skull while performing a craniotomy. Removal of parts of the skull allows access to areas of the brain, and careful manipulation of tissue has even allowed access to structures deep within the brain. Recently, magnetic resonance imaging (MRI) has proved a useful method for viewing structures of the brain and has been included in some such procedures. However, one difficulty that has arisen is that, while the MRI can provide excellent images, the bulky MRI machines in use do not allow the surgeon to perform a procedure while viewing a changing or real-time image. Further, access to deeper structures creates a variety of risks caused by the manipulation of the soft tissues of the brain. An alternative to craniotomy could help with these difficulties.
One alternative method of accessing the inner areas of the brain that has recently arisen is through vascular catheterization and advancement into areas of the brain. The methods enable different treatment and analysis options that may supplement or replace those offered by craniotomy. However, these methods are limited to areas accessible using vascular passageways. Further, with respect to introducing drugs or other substances, the transfer of some substances, for example some proteins, across the barriers between blood and the cerebrospinal fluid (CSF), is not allowed by the biological mechanisms that control the interaction of blood and CSF.
The subarachnoid space is a compartment that contains the body of the spinal cord and CSF, which is a fluid that fills and surrounds the ventricles of the brain and the spinal cord and acts as a lubricant and a mechanical barrier against shock. The spinal subarachnoid space is the part of the subarachnoid space containing the spinal cord and extending into the base of the brain cavity. This space may represent a viable nonvascular route to accessing the brain, providing an alternative to vascular catheterization and/or craniotomy. One method of using the subarachnoid space would be to introduce a catheter into the subarachnoid space and advance it to a chosen location along the spinal column, or, potentially, to enter the cranium and access the brain. Such methods are noted and discussed, along with a variety of potential treatment options, in U.S. patent application Ser. No. 09/905,670 entitled METHODS AND APPARATUSES FOR NAVIGATING THE SUBARACHNOID SPACE, which is expressly incorporated herein by reference.
With respect to catheterization, the MRI machine may create difficulties for procedures that would be desirable, for example, once the spinal subarachnoid space is accessed, it may be necessary to change catheters. To change catheters could require removing the patient from the MRI machine, increasing the time and difficulty of the overall operation. To make such changes easier, it would be helpful to use an introducer sheath that would enable several catheters to be introduced at once, or to enable a catheter to be removed and a different catheter to be introduced at the same location. However, conventional introducer sheaths known for use in vascular procedures often extend a very short distance outside the patient's body, and so would be difficult to access while the patient, and hence the proximal end of the introducer sheath, is in an MR imaging device.
An introducer sheath for use with entry into the spinal subarachnoid space should provide access past the bones of the spine. Introducer sheaths for use in vascular catheterization are designed to enter arteries not protected by such protective bony surroundings. New introducer sheath geometries may be useful to address these problems.